I contacted Tokyo Skydiving Club a couple of years ago on firstname.lastname@example.org. They were super friendly, helping out with all the hoops you need to jump through to go skydiving in Japan. IIRC you need to be part of their national association and have local insurance. They offered to fill in all the papers and everything. The only thing I had to do was go to a local post office once in Japan to pay for the insurance. I never ended up going though so I couldn't say anything about the DZ as such. They even offered to pick me up from the train, so I'm sure the atmosphere has to be great.